I. Field of the Invention
This invention relates to a fourth generation medical computerized tomographic (CT) apparatus. This invention particularly relates to a fourth generation medical CT apparatus which is capable of reconstructing a section of an image from data produced by a partially rotated source having a fan-shaped beam of radiation energy.
II. Background of the Invention
Fourth generation medical CT apparatus are known in which a source of radiation is rotated about an object being scanned. For example, in FIG. 1 a source of radiation 10 is shown rotated counterclockwise in an orbit 12 from a first position Si. Within the arc defined by rotation of source 10 is a scanning circle 14 within which an object under observation may be placed.
Source 10 generates a narrow fan-beam of radiation which has an apex or source fan angle of 2a as shown in FIG. 1. Source fan angle 2a is preferably chosen so that the resultant fan-beam from source 10 completely encompasses scanning circle 14.
Detectors 16 are typically located around a circle cocentered with scanning circle 14. As source 10 rotates the resultant source fan-beam strikes selected groups of detector elements 16. When rotation of source 10 is completed, data collected from each detector element 16 may be reordered and collected in data sets which each define a detector fan-beam. A detector fan-beam of data, therefore, comprises data collected for any particular detector element 16 as the source fan-beam from source 10 passes by that detector element.
In a third generation medical CT apparatus both the source and detectors simultaneously rotate around the object being scanned. Such an apparatus is, for example, disclosed in U.S. Pat. No. 4,075,492 issued to Boyd et al. In the Boyd et al. apparatus divergent rays from the rotating source strike the rotating detectors. The resultant divergent detector fan-beam data is reordered into parallel ray detector fan-beam data format. Parallel ray data format is data in a format corresponding to the data which would have been received if parallel rays of radiation had passed through the scanning circle, instead of the divergent rays from the rotating source. The parallel ray data is then subjected to convolution and back projection in Boyd et al. to reconstruct an image of the object under observation.
Another reconstruction system using fan-beams is disclosed in an article by A. B. Lakshminarayanan entitled "Reconstruction From Divergent Ray Data," Technical Report No. 92, State University of New York at Buffalo, Department of Computer Science, January 1975. This article suggests reconstruction of images without reordering the divergent detector fan-beam data into parallel ray data. This method of image reconstruction has the obvious advantage of avoiding the difficulty of determining parallel ray data from divergent ray fan-beam data. The Lakshminarayanan method performs convolution and back projection without such reordering and this method is sometimes referred to as a direct fan-beam reconstruction method.
Methods of image reconstruction such as those taught by Boyd et al. and Lakshminararyanan contemplate complete rotation of the source of radiation about the object being scanned. In this regard such CT apparatus are superior in contrast resolution to conventional x-ray apparatus using an x-ray film or TV camera. This improved contrast resolution permits low contrast differences of various organs to be distinguished. Thus in CT apparatus of this type soft tissues of organs can be clearly observed.
However, CT apparatus of this type are inferior in time resolution to conventional x-ray apparatus. Time resolution is determined by how rapidly data necessary for image reconstruction can be obtained. Complete rotation of a source about an object being scanned takes a considerable amount of time and as a consequence artifacts in the resultant images may be caused by motion of the object under observation during such rotation. Accordingly, efforts were directed toward shortening the fan-beam rotation time in order to reduce these artifacts. For example, with reference to FIG. 1 source 10 may be rotated from position Si a total of .pi. radians (180 degrees) plus the source fan angle 2a (which is less than 180 degrees) until source 10 reaches a second position Sx. Given this less than 360 degrees of rotation, full detector fan data will not be received for all of detectors 16.
For the geometries illustrated in FIG. 1, only those detectors within arc 18 will receive full detector fan data. Those detectors within arcs 20 will receive partial detector fan data and those detectors within arc 22 will receive no detector fan data. Specifically, with 180 degrees plus source fan angle 2a of rotation of source 10, detector 16i, which is in an incomplete detector fan arc 20, is missing detector fan data which, if source 10 were rotated a full 360 degrees, would be obtained while source 10 rotated from position Sa to position Si.
With 180 degrees plus the source fan angle 2a of rotation, the resultant detector fan data is duplicative for some projections through scan circle 14 and not duplicative for other projections. More specifically, again referring to detector 16i when source 10 is in position Sb a ray of energy 24 would pass through unit volume X.sub.1 within scan circle 14 and reach detector 16i. When source 10 is moved to position Sc a ray of energy 26 from source 10 will pass through unit volume X.sub.1 and strike detector 16j thereby giving duplicative information for a projection through circle 14 including unit volume X.sub.1, assuming no motion occurred.
However, for unit volume X.sub.2 detector 16i receives no data for any location of source 10 between positions Si and Sx, rendering the data for such a projection through unit volume X.sub.2 nonduplicative and, therefor, distinguishable from the data accumulated for unit volume X.sub.1. In other words unit volume X.sub.1 has overlapping data to reconstruct a projection of that unit volume, but unit volume X.sub.2 has only minimal data. Exact image reconstruction is difficult to obtain without any compensation for such nonuniformity of data sampling within scanning circle 14.
U.S. Pat. No. 4,284,896 issued to Stonestrom teaches the technique to provide nonuniform data compensation in a third generation CT apparatus. In this technique, single entry data is duplicated or reflected to provide overlapping data for all detector fan beams, even though the source fan beam is rotated only 180 degrees plus the source fan angle. Thus all the unit volumes within the scanning circle are sampled by rays as though the source were rotated a full 360 degrees. However, this technique requires excess time to generate the additional data by duplicating or reflecting obtained data. Also errors are likely to occur in the duplication or reflection process.
Another compensation technique for less than 360 degree source rotation is shown in U.S. Pat. No. 4,293,912 issued to Walters. In this technique data for any particular unit volume in the scanning circle is limited to data from projections which range 180 degrees about that unit volume. The rest of the data regarding projections through that unit volume is filtered out prior to convolution and back projection. For example, if unit volume X.sub.1 of FIG. 1 were considered, only projections starting from that generated by ray 24 and continuing 180 degrees until the projection obtained from ray 26 might be considered. Any additional projections through unit volume X.sub.1 would be disregarded.
This technique, however, is subject to the occurrence of motion artifacts. Although antiparallel but collinear rays, such as rays 24 and 26 in FIG. 1, sample the same projection in scanning circle 14, their data values may be different because of the time difference when they were obtained. Any patient motion during this interval will result in image artifacts. The Walters technique does not allow the possibility of reducing the negative effects of such motion artifacts since redundant data are removed before image reconstruction.
An additional technique is shown in an article by Abraham Naparstek entitled "Short-Scan Fan-Beam Algorithms for CT", IEEE Transactions on Nuclear Science, Volume NS-27, No. 3, June 1980, and in articles by Dennis L. Parker entitled "Optimal Short Scan Convolution Reconstruction For Fanbeam CT", Med. Phys. 9(2), March/April 1982, pages 254-257 and "Optimization of Short Scan Convolution Reconstruction In Fanbeam CT", Department of Radiation Oncology, University of California at San Francisco, 1982, pages 199-202. Naparstek discloses several short-scan reconstruction algorithms of the convolution type for fan-beam projections. Parker demonstrates a reconstruction method for limited angle source rotation as applied to third generation CT apparatus wherein both the source of fan-beam radiation and the detector assembly receiving fan-beam radiation from the source rotate about the object. Parker discloses a weighting scheme which requires that the sum of the two weights corresponding to the same line-integral must equal one in regions for incomplete data collection. Single and double scanning occurs in third generation CT scanners of the type for which the Parker methodology is applicable. However, in third generation CT apparatus each detector fan is itself complete. In fourth generation CT apparatus each detector fan is not necessarily complete.
Moreover, in a fourth generation CT apparatus, in which only the fan-beam radiation source rotates around the object and the detector assembly surrounding the object remains stationary, source fan-beams are usually sorted into detector fan-beams divergent from each element in the detector assembly in order to increase the fan-beam ray density. The fan-beam ray density of a source fan depends on the detector element separation. This quantity is difficult to make small. However, the density of detector fan rays is easily increased by making the angle between source fans small. High density of the fan-beam rays contribute to high spacial resolution of the image.
When a third generation CT apparatus technique is applied to a fourth generation CT apparatus technique, it appears necessary to obtain complete detector fans for 180 degrees plus the detector fan angle. The source rotation required in this case would be 180 degrees plus twice the detector fan angle plus the source fan angle. This increased source rotation would seriously compromise the minimum resolutions time for dynamic CT scans. Furthermore, neither Naparstek nor Parker derive an exact reconstruction method, instead they present only an approximation. In addition, the Naparstek and Parker convolution functions are restricted to a discrete function form. This discrete function has oscillating values instead of even and/or odd arguments. Accordingly, it is difficult to expect such convolution to result in smooth images.
Accordingly, an object of the subject invention is to reduce motion artifacts in a fourth generation CT apparatus while permitting limited source rotation on the order of 180 degrees plus the source fan angle.
A further object of the present invention is to provide a convolution for smoothing the resultant image using an even function to achieve such a result.
In short, the subject invention has as a primary object a new and improved fourth generation CT apparatus which overcomes the above-mentioned problems of the known prior art and provides image reconstruction with a high degree of quality.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention.